Tfa image sensor with stability-optimized photodiode

ABSTRACT

The invention relates to a TFA image sensor with stability-optimized photodiode for converting electromagnetic radiation into an intensity-dependent photocurrent with an intermetal dielectric, on which, in the region of the pixel matrix, a lower barrier layer is situated and a conductive layer is situated on the barrier layer, and vias being provided for the contact connection to the ASIC, the vias in metal contacts on the ASIC. A TFA image sensor having improved electrical properties is provided. This is achieved in that an intrinsic absorption layer is provided between the TCO layer and the barrier layer with a layer thickness of between 300 nm and 600 nm. Before the application of the photodiodes, the topmost, comparatively thick metal layer of the ASIC is removed and replaced by a matrix of thin metal electrodes which form the back electrodes of the photodiodes, the matrix being patterned in the pixel raster.

The present patent application relates to a TFA image sensor withstability-optimized photodiode for converting electromagnetic radiationinto an intensity-dependent photocurrent with an intermetal dielectric,on which, in the region of the pixel matrix, a lower barrier layer(metal 2) is situated and a conductive layer (metal 2) is situated onsaid barrier layer, and vias being provided for the contact connectionto the ASIC, said vias in metal contacts on the ASIC.

Such a TFA sensor (Thin Film on ASIC (TFA) Technology) comprises amatrix-organized or linear arrangement of pixels. The electroniccircuits for operating the sensor (e.g. pixel electronics, peripheralelectronics, system electronics) are usually realized using CMOS-basedsilicon technology and form an application specific integrated circuit(ASIC).

Isolated therefrom by an insulating layer and connected thereto by meansof corresponding electrical contacts, there is situated on the ASIC amultilayer arrangement as photodiode, which performs the conversion ofelectromagnetic radiation into an intensity-dependent photocurrent. Saidphotocurrent is transferred at specific contacts—present in eachpixel—of the pixel electronics underneath (B. Schneider, P. Rieve, M.Böhm, Image Sensors in TFA (Thin Film on ASIC) Technology, ed. B. Jähne,H. Hausecker, P. Geiβler, Handbook of Computer Vision and Applications,pp. 237-270, Academic Press, San Diego, 1999).

According to the prior art (J. A. Theil, M. Cao, G. Kooi, G. W. Ray, W.Greene, J. Lin, A J. Budrys, U. Yoon, S. M a, H. Stork, HydrogenatedAmorphous Silicon Photodiode Technology for Advanced CMOS Active PixelSensor Imagers, MRS Symposium Proceedings, vol. 609, 2000), what is usedas photodiode is a pin configuration based on amorphous silicon, i.e. asequence comprising a p-conducting, an intrinsically conducting(intrinsic) and an n-conducting amorphous silicon layer. The n-typelayer usually forms the bottom most layer facing the ASIC.

The electrical contacts are formed by a metal layer, for example, onsaid side facing the ASIC, while the contact connection on the sidefacing the direction of light incidence is generally effected by atransparent and conductive layer.

Over and above the pin photodiode mentioned, further componentstructures are also possible, e.g. Schottky photodiodes, in which anintrinsic semiconductor layer is brought into contact with a suitablemetal (for example chromium, titanium, platinum, palladium, silver), sothat the metal-semiconductor junction forms a Schottky photodiode.

A typical layer configuration is disclosed in the patent application TFAimage sensor with extremely low dark current (file reference10063837.6). Furthermore, detector structures with a controllablespectral sensitivity are known (P. Rieve, M. Sommer, M. Wagner, K.Seibel, M. Böhm, a-Si:H Color Imagers and Colorimetry, Journal ofNon-Crystalline Solids, vol. 266-269, pp. 1168-1172, 2000). This basicstructure of a TFA image sensor can furthermore be extended byadditional, upstream layers in the direction of light incidence, forexample by color filter layers (e.g. Bayer pattern, U.S. Pat. No.3,971,065).

If amorphous silicon is used as photoactive sensor material, then themetastability observed in the case of this material becomes apparent,under certain circumstances. Hydrogenated amorphous silicon (a-Si:H)comprises a silicon-hydrogen atomic composite lacking a long-range orderas is typical of semiconductor crystals. Modifications of the atombonding parameters occur with respect to the ideal semiconductorcrystal. The consequence of this is that, in the context of thesolid-state band model, a state density that differs from zero exists inthe band gap between conduction band and valence band, which affects theelectrical and optical properties of the material. States in the middleof the band gap predominantly act as recombination centers, while statesin the vicinity of the band edges function as traps for charge carriers.On account of light being radiated in or injection of charge carriers,more precisely through recombination of injected charge carriers, weaksilicon bonds are broken and additional band gap states arise.

These band gap states caused by light irradiation represent additionalrecombination or trapping centers and influence the charge carriertransport and the distribution of the electric field strength in thecomponents fabricated from amorphous silicon. In pin photodiodes, forexample, predominantly positively charged states are concentrated intraps in that region of the intrinsic layer (i-type layer) which adjoinsthe p-type layer, and negatively charged states in that region of thei-type layer which adjoins the n-type layer. These stationary chargesresult in a decrease in the magnitude of the electric field strengthwithin the i-type layer, so that the accumulation of photogeneratedcharge carriers deteriorates. An efficient charge carrier accumulationin pin photodiodes made of amorphous silicon is provided when the driftlength (μτE) of the charge carriers significantly exceeds the thicknessd of the intrinsic layer:μτE>>d   (1)

On account of the increase in the defect density associated with lightbeing radiated in, on the one hand the lifetime τ is reduced due tointensified recombination of charge carriers and on the other hand theelectric field E is reduced on account of the charged states in thei-type layer. Both have the consequence that the ratio of drift lengthto i-type layer thickness is reduced and the photocurrent thusdecreases. The decrease in the photocurrent becomes apparentparticularly when the photodiode is operated near the short-circuitpoint without additional reverse voltage, i.e. when only the built-inpotential difference brought about by the doped layers is effective.

When reverse voltage is applied, by contrast, the electric fieldintensifies, so that the charge carrier accumulation is impaired to aless extent. The essential consequence of the light irradiation of aphotodiode made of amorphous silicon with regard to the photocurrentthus consists in a reduction of the photocurrent saturation.

The dark current of an a-Si:H photodiode, i.e. the current which flowseven in the unilluminated state is likewise influenced by thedegradation of the material. On account of the defect statesadditionally generated by light being radiated in, the thermalgeneration of charge carriers increases in the case of a reverse-biasedphotodiode (extraction), which is manifested in an increase in the darkcurrent.

The invention is now based on the object of providing a TFA image sensorwith stability-optimized photodiode for converting electromagneticradiation into an intensity-dependent photocurrent having improvedelectrical properties.

The formulated object on which the invention is based is achieved, inthe case of a TFA image sensor with stability-optimized photodiode forconverting electromagnetic radiation into an intensity-dependentphotocurrent, in that a layer thickness of the intrinsic absorptionlayer of between 300 nm and 600 nm is provided.

Further refinements of the invention emerge from the associatedsubclaims.

The object on which the invention is based is furthermore achieved bymeans of a method which is characterized in that, before the applicationof the photodiodes, the topmost, comparatively thick metal layer of theASIC is removed and replaced by a matrix of thin metal electrodes whichform the back electrodes of the photodiodes, said matrix being patternedin the pixel raster.

Further refinements of the method according to the invention emerge fromthe associated subclaims.

One particular refinement of the invention is characterized by openingof the ASIC passivation in the photoactive region of the TFA sensor,removal of the antireflection layer of the upper metalization layer ofthe ASIC in the photoactive region of the TFA sensor, removal of theconductive layer of the upper metalization layer of the ASIC in thephotoactive region of the TFA sensor, patterning or removal of the lowerbarrier layer of the upper metalization layer of the ASIC in thephotoactive region of the TFA sensor, deposition and patterning of afurther metal layer, deposition and patterning of the photodiode layers,and deposition and patterning of further layers, such as color filterlayers.

The changes in the dark current and photocurrent brought about by lightbeing radiated in are reduced, according to the invention, by reducingthe thickness of the intrinsic layer. This measure brings about anincrease in the electric field strength over the i-type layer, so thatthe field strength depth caused by the increase in the defect density onaccount of light being radiated in, within the i-type layer, is lesssharply pronounced.

In this way, the accumulation condition for photogenerated chargecarriers which is given by equation (1) can be met even in the state ofincreased defect density (after light has been radiated in), and adecrease in photosensitivity is avoided. With regard to the behavior ofthe photodiode without illumination, photodiodes with a small i-typelayer thickness, in the aged state, have a lower dark current than thosewith a thick i-type layer, which can be attributed to the smaller numberof generation centers present in the band gap.

The method of improving the stability of photodiodes made of amorphoussilicon by means of a thin absorber layer is known from the field ofphotovoltaic technology, where it is employed successfully in solarcells based on amorphous silicon. Application to image sensors in TFAtechnology is novel. The method is suitable both for photodiodes of thepin or nip type and for Schottky diodes. A layer thickness of theintrinsic absorption layer of between 300 nm and 600 nm has proved to beadvantageous with regard to the stability of the photodiode, and itshould preferably be approximately 450 nm.

One advantageous development consists in increasing the band gap of theintrinsic absorber layer of the photodiode. The dark current can bereduced in this way. At the same time, it is possible to counteract theincrease in the diode capacitance which accompanies the reduction of thei-type layer thickness. Technologically, it is possible to increase theband gap for example by using an amorphous silicon-carbon alloy(a-SiC:H) as absorption layer.

In photodiodes with a small i-type layer thickness, the configuration ofthe surface on which the diode is situated is of crucial importance forthe magnitude of the dark current. Besides the thermal generationcurrents already mentioned, inhomogeneities of the ASIC surface form,caused by the structures (metal tracks, holes in passivation layer,etc.) situated thereon, a further source of undesirably high darkcurrents in TFA image sensors. In this case, the influence of thesurface topography is greater, the thinner the photodiode situatedthereon. In this respect, it is necessary in particular to deposit thephotodiode of reduced layer thickness on a surface that is as planar aspossible.

One advantageous development of the invention thus consists indepositing the photodiode with small i-type layer thickness (asmentioned above) on an ASIC having a flat surface topography. This isensured by the fabrication process explained below. The ASIC can, butneed not necessarily, be coated with a passivation.

Within the pixel matrix, firstly the back electrodes of all the pixelsare connected to one another via the topmost CMOS metal plane, which ismade planar in the region of the pixel matrix. This metal area issituated on a CMP-planarized surface (CMP=Chemical Mechanical Polishing)of the topmost intermetal dielectric layer. Before the application ofthe photodiodes, this topmost, comparatively thick metal layer of theASIC is removed and replaced by a matrix of thin metal electrodes whichform the back electrodes of the photodiodes, said matrix being patternedin the pixel raster. The topmost metallization of the ASIC generallycomprises a multilayered arrangement comprising a lower barrier layer,e.g. titanium nitride or titanium, the actual conductive layer, e.g.aluminum (alloys) and, if appropriate, an upper antireflection layer,e.g. titanium nitride. In an expedient manner, the antireflection layer(if present) and the metal layer are completely removed above the pixelmatrix, so that all that remains is the lower barrier layer. The latteris then patterned in the pixel raster and either forms the pixel backelectrode directly, or it is coated with a further metal layer, e.g.chromium, which forms the matrix of the pixel back electrodes after afurther patterning step. As an alternative, the lower barrier layer iscompletely removed, this then being followed by the deposition andpatterning of the further metal layer in the form of pixel backelectrodes.

The process steps are summarized below as key points:

a) if appropriate opening of the ASIC passivation in the photoactiveregion of the TFA sensor,

b) if appropriate removal of the antireflection layer of the uppermetallization layer of the ASIC in the photoactive region of the TFAsensor,

c) removal of the conductive layer of the upper metallization layer ofthe ASIC in the photoactive region of the TFA sensor,

d) patterning or removal of the lower barrier layer of the uppermetallization layer of the ASIC in the photoactive region of the TFAsensor,

e) if appropriate deposition and patterning of a further metal layer,

f) deposition and patterning of the photodiode layers,

g) if appropriate deposition and patterning of further layers (e.g.color filter layers).

In this way, a largely planar surface is ensured in the region of theactive pixel matrix of the sensor because the etching attack into thetopmost intermetal dielectric layer is reduced to a minimum. It is onlyduring the patterning or removal of the lower barrier layer of thetopmost ASIC metallization layer that the CMP-planarized dielectriclayer is uncovered and is removed locally by the etching attack, whichcan be minimized by a suitable choice of process parameters. Apart fromthat, the flat surface topography of the CMP planarization ismaintained, thus avoiding any influencing of the dark current of thephotodiodes deposited thereon.

The invention is explained below with reference to some drawings. FIGS.1 and 2 show a pin and, respectively, a Schottky photodiode with anintrinsic absorption layer i according to the invention made ofamorphous silicon in the layer thickness range of between 300 nm and 600nm. The following figures relate to the abovementioned fabricationprocess which ensures a largely planar surface topography.

In this case, the illustrations only include the topmost layers of theASIC which are relevant to the interface with the TFA layers.

FIG. 3 illustrates the initial state before the beginning of the TFAprocessing in the form of a passivated ASIC with passivation that hasbeen opened in the region of the pixel matrix. The antireflection layerof the topmost metallization layer of the ASIC is likewise removed inthe pixel region.

In this case, firstly an intermetal dielectric is arranged on the ASICand, in the region of the pixel matrix, a lower barrier layer (metal 2)is situated on said intermetal dielectric and a conductive layer (metal2) is situated on said barrier layer. Vias are provided for the contactconnection to the ASIC, said vias ending in metal contacts on the ASIC.Furthermore, a bond pad (metal 2) for external contact connection isprovided, which is contact-connected to the ASIC by means of vias and ametal 1.

The state after the removal of the conductive layer of the topmostmetallization is recorded in FIG. 4.

FIG. 5 documents the result after the patterning of the lower barrierlayer. This produces the pixel back electrodes, which are subsequentlycoated with the multilayer system comprising amorphous silicon and TCO(FIG. 6). FIGS. 7 and 8 show a process variant in which, proceeding fromthe situation according to FIG. 5, the patterned regions of the lowerbarrier layer are covered by a further patterned metal layer before thedeposition of the photodiode. Beginning with FIG. 9, a further variantis illustrated in which, after the situation outlined in FIG. 4, thelower barrier layer of the topmost metallization layer of the ASIC iscompletely removed. The further metal layer is subsequently depositeddirectly onto the intermetal dielectric, and forms the pixel backelectrodes after patterning (FIG. 10). The photodiode-forming layers arethen applied thereto (FIG. 11).

1. A TFA image sensor with stability-optimized photodiode for convertingelectromagnetic radiation into an intensity-dependent photocurrent withan intermetal dielectric, on which, in the region of the pixel matrix, alower barrier layer is situated and a conductive layer is situated onsaid barrier layer, and vias being provided for the contact connectionto the ASIC, said vias ending in metal contacts on the ASIC, wherein anintrinsic absorption layer is provided between the TCO layer and thebarrier layerwith a layer thickness of between 300 nm and 600 nm.
 2. TheTFA image sensor as claimed in claim 1, wherein the layer thickness ofthe intrinsic absorption layer is approximately 450 nm.
 3. The TFA imagesensor as claimed in claim 1, wherein the band gap of the intrinsicabsorption layer of the photodiode is increased.
 4. The TFA image sensoras claimed in claim 1, wherein the increase in the band gap is realizedby using an amorphous silicon-carbon alloy (a-Sic:H) as absorptionlayer.
 5. The TFA image sensor as claimed in claim 1, wherein, inparticular, the photodiode of reduced layer thickness is arranged on asurface that is as planar as possible.
 6. The TFA image sensor asclaimed in claim 1, wherein the photodiode with small intrinsic layerthickness is deposited on an ASIC having a flat surface topography. 7.The TFA image sensor as claimed in claim 1, wherein the ASIC is coatedwith a passivation.
 8. The TFA image sensor as claimed in claim 1,wherein, within the pixel matrix, firstly the back electrodes of all thepixels are connected to one another via the topmost CMOS metal plane,which is made planar in the region of the pixel matrix.
 9. The TFA imagesensor as claimed in claim 8, wherein the metal plane is situated on aCMP-planarized surface (CMP=Chemical Mechanical Polishing) of thetopmost intermetal dielectric layer.
 10. A method for fabricating a TFAimage sensor as claimed in claim 1, wherein, before the application ofthe photodiodes, the topmost, comparatively thick metal layer of theASIC is removed and replaced by a matrix of thin metal electrodes whichform the back electrodes of the photodiodes, said matrix being patternedin the pixel raster.
 11. The method as claimed in claim 10, wherein anantireflection layer that is present and the metal layer are completelyremoved above the pixel matrix, so that all that remains is the barrierlayer situated underneath.
 12. The method as claimed in claim 10,wherein the lower barrier layer is completely removed, this then beingfollowed by the deposition and patterning of the further metal layer inthe form of pixel back electrodes.
 13. The method as claimed in claim10, wherein the ASIC passivation is opened in the photoactive region ofthe TFA sensor.
 14. The method as claimed in claim 10, wherein theantireflection layer of the upper metallization layer of the ASIC in thephotoactive region of the TFA sensor is removed.
 15. The method asclaimed in claim 10, wherein the conductive layer of the uppermetallization layer of the ASIC in the photoactive region of the TFAsensor is removed.
 16. The method as claimed in claim 10, wherein thelower barrier layer of the upper metallization layer of the ASIC in thephotoactive region of the TFA sensor is patterned or removed.
 17. Themethod as claimed in claim 16, wherein a further metal layer isdeposited and patterned.
 18. The method as claimed in claim 17, whereinfurther layers, such as color filter layers, are deposited andpatterned.
 19. A method for fabricating a TFA image sensor as claimed inclaim 1, wherein the ASIC passivation in the photoactive region of theTFA sensor is opened, the antireflection layer of the uppermetallization layer of the ASIC in the photoactive region of the TFAsensor is removed, the conductive layer of the upper metallization layerof the ASIC in the photoactive region of the TFA sensor is removed, thelower barrier layer of the upper metallization layer of the ASIC in thephotoactive region of the TFA sensor is patterned or removed, a furthermetal layer is deposited and patterned, the photodiode layers aredeposited and patterned, and further layers, such as color filterlayers, are deposited and patterned.